Waste-to-energy (WtE) also referred to as energy-from-waste (EfW) is a type of waste incineration/combustion process in which boilers or other heat recovery systems are used to recover the energy from the combustion gases for use in power or heat generation. Most WtE processes produce electricity directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol or synthetic fuels.
The heat produced by an incinerator can be used to generate steam which may then be used to drive a turbine in order to produce electricity. The typical amount of net energy that can be produced per tonne municipal waste is about 2/3 MWh of electricity and 2 MWh of district heating. Thus, incinerating about 600 metric tons per day of waste will produce about 400 MWh of electrical energy per day (17 MW of electrical power continuously for 24 hours) and 1200 MWh of district heating energy each day.
Energy can be recovered from waste by various (very different) technologies. It is important that recyclable material is removed first, and that energy is recovered from what remains, i.e. from the residual waste. Energy from waste (EfW) technologies include:
Incineration in which the residual waste burns at 850°C and the energy recovered as electricity or heat.
Gasification and pyrolysis, where the fuel is heated with little or no oxygen to produce “syn-gas” which can be used to generate energy or as a feed-stock for producing methane, chemicals, bio-fuels, or hydrogen (see also landfill gas and sewage gas)
Anaerobic digestion, which uses microorganisms to convert organic waste into a methane-rich bio-gas that can be combusted to generate electricity and heat or converted to bio-methane. This technology is most suitable for wet organic wastes or food waste. The other output is a biofertiliser.
Incineration
Incineration, the combustion of organic material such as waste with energy recovery is the most common WtE implementation. The method of using incineration to convert municipal solid waste (MSW) to energy is a relatively old method of waste-to-energy production. Incineration generally entails burning a Refuse Derived Fuel (RDF) to boil water which powers steam generators that make electric energy to be used in homes and businesses.
One problem associated with incinerating MSW to make electrical energy, is the potential for pollutants to enter the atmosphere with the flue gases from the boiler. These pollutants can be acidic and in the 1980s were reported to cause environmental damage by turning rain into acid rain. Since then, the industry has removed this problem by the use of lime scrubbers and electro-static precipitators on smokestacks.
The limestone mineral used in these scrubbers has a pH of approximately 8 which means it is a base. By passing the smoke through the lime scrubbers, any acids that may be in the smoke are neutralized which prevents the acid from reaching the atmosphere and hurting the environment.
Gasification and Pyrolysis
Gasification, as applied to solid waste materials and biomass, is a relatively new application of this technology that is increasingly being used for the disposal of wastes. It is a thermo-chemical process in which wastes, including their biomass content is heated, in an oxygen deficient environment to produce a low-energy gas containing hydrogen, carbon monoxide and methane. The gas can then be used as a fuel in a turbine or combustion engine to generate electricity.
Gasifiers fueled by fossil sources such as coal have been operating successfully for many years, but they are now increasingly being developed to accept more mixed fuels, including wastes. New gas clean-up technology ensures that the resulting gas is suitable to be burnt in a variety of gas engines, with a very favorable emissions profile. Gasifiers can operate at a smaller scale than an incineration plant, and can also be provided in modular form to suit a range of different scales of operation. A number of British companies are leading in this emerging technology.
Pyrolysis is another emerging technology, sharing many of the characteristics of gasification. With gasification partial oxidation of the waste occurs, whilst with pyrolysis the objective is to heat the waste in the complete absence of oxygen. Gas, olefin liquid and char are produced in various quantities. The gas and oil can be processed, stored and transported, if necessary and combusted in an engine, gas turbine or boiler. Char can be recovered from the residue and used as a fuel, or the residue passed to a gasifier and the char gasifed.